This invention relates to a magnetron for a microwave oven. More specifically, this invention concerns the improvements in the structure of a high-voltage input terminal which is secured to the shield box of a magnetron.
In magnetrons for microwave ovens, a negative high voltage to be given to the filament cathode is applied to the cathode input stem, to which power for heating is also supplied. In a magnetron for use in a microwave oven for household use, for example, the anode voltage is 4 kV, the anode current is 300 mA, the filament voltage is 3.15 V, and the filament current is 10 A. For high-voltage terminals used to introduce high voltages and large currents such as mentioned above, a feed-through-type high-voltage capacitor structure, in which a ceramic high dielectric element is built, is generally adopted. In high-voltage terminals of this structure which serves as the input terminal, a feed-through-type high-voltage capacitor has an electrostatic capacity of several hundreds of pF, and forms an LC filter in combination with a choke coil to thereby suppress noise that is generated by the magnetron.
The dielectric characteristics of the high dielectric element, to which a high voltage is applied, is considered particularly important to a magnetron. Generally, the high dielectric element is covered by an insulating resin to protect the interface. This adds necessary steps to the production process. Moreover, it is considered very difficult to implement quality control in terms of dielectric strength. For this reason, efforts are being made to simplify the structure of the high-voltage input terminal insofar as possible. There are limitations in simplification of the structure of the high-voltage input terminal so long as high dielectric elements are used. A possible very simplified example is a high-voltage input terminal in a structure that can withstand high voltages by use of a thin layer of insulating resin and also has an electrostatic capacity.
Molding can be used for integration of a plurality of electrodes with an insulating resin, which are the component parts of a high-voltage input terminal. However, since the thermal expansion coefficient of an electrode metal is smaller than that of an insulating resin, there is a possibility that the resin cracks when it is subjected to a heat cycle, etc. This possibility is high particularly when the resin is subjected to a tensile stress. In magnetrons for electronic ranges, above all, since the choke coil which is an input conductor and the shield box are raised to high temperatures when in service, the input terminal connected to them is sometimes heated to 100.degree. C. Therefore, the resin used in the input terminal of a magnetron for a microwave oven is required to have a sufficient heat resistance. When the insulating resin portion is formed by molding, if the insulating resin is used in a thin layer to provide the high-voltage input terminal with an adequate electrostatic capacity, it is difficult to maintain the structure of the insulating resin after the molding in good quality, from a microscopic point of view. Also, it is relatively difficult to make the insulating resin structure withstand mechanical stresses and prevent the occurrence of voids to ensure that the dielectric performance of the insulating resin itself can be exhibited to the full.
There are known structures of the high-voltage input terminal, which are disclosed, for example, in Japanese Utility Model Disclosure Nos. 48-5652, 50-58634, 60-126963 and 60-129058. However, it has been found that if an attempt is made to obtain necessary electrostatic capacity and high dielectric strength only through elaborate contrivances about the structure of the insulating resin layer, the resin layer of uniform quality cannot be realized from a microscopic point of view, because the arrangement of the electrode parts inevitably becomes complicated and, therefore, the flow of resin during the molding becomes complex.